I want to see a planetary transit. How can I polar align my telescope in the daytime?
One good way is to use the Sun. Carefully level your mount with a bubble level and set the polar axis to the latitude of your site. Hang a weighted string from the mount (between the tripod legs) and lay a protractor on the ground,centered under the string. Rotate the protractor until the string’s shadow points to the Sun’s known azimuth (measured from north through east) plus 180°. Finally, swivel the mount until the polar axis stands directly above 0° on the protractor. You are now polar-aligned.
To obtain the Sun’s azimuth, enter the date, time, latitude, and longitude of your site into a planetarium program. Even better, make your own “protractor” using our Interactive Sky Chart. You’ll need several of these all-sky charts, preprinted for your planned viewing site. Make them for convenient times 10 minutes apart. Each sheet will show the Sun, and you can draw the north-south line (from the middle of the label “north” through the middle of the label “south”). A line from the Sun through the exact center of the north-south line gives you the direction of the shadow it will cast. Since these sky charts were designed for looking up at the sky, they have to be placed face down to make north, south, east, and west correspond to directions on the ground. So transfer the north-south line, center point,and direction of the shadow to the back of the sheet for use as a scope-aligning aid. (Put the sheet up against a windowpane and trace.)
This alignment method works any time of day, unless the Sun is very high overhead.
— Edwin L. Aguirre